Magnetic resonance imaging (“MRI”) has been developed as an imaging technique adapted to obtain both images of anatomical features of human patients as well as some aspects of the functional activities and characteristics of biological tissue. These images have medical diagnostic value in determining the state of health of the tissue examined. Unlike the situation with fluoroscopic imaging, a patient undergoing magnetic resonance imaging procedure may remain in the active-imaging system for a significant amount of time, e.g. a half-hour or more, without suffering any adverse effects.
In an MRI process, a patient is typically aligned to place the portion of the patient's anatomy to be examined in the imaging volume of the MRI apparatus. Such an MRI apparatus typically comprises a primary magnet for supplying a constant magnetic field (B0) which, by convention, is along the z-axis and is substantially homogeneous over the imaging volume and secondary magnets that can provide linear magnetic field gradients along each of three principal Cartesian axes in space (generally x, y, and z, or x1, x2 and x3, respectively). As is known to those skilled in the art, a magnetic field gradient (ΔB0/Δxi) refers to the variation of the field with respect to each of the three principal Cartesian axes, xi. The MRI apparatus also comprises one or more RF (radio frequency) coils which provide excitation and detection of the MRI signal. Additionally, or alternatively, detection coils may be designed into the distal end of a catheter to be inserted into a patient. When such catheters are employed, their proximal ends are connected to the receiving signal input channel of the magnetic resonance imaging device. The detected signal is transmitted along the length of the catheter from the receiving antenna and/or receiving coil in the distal end to the MRI input channel connected at the proximal end. Other components of an MRI system are the programmable logic unit and the various software programs which the programmable logic unit executes. Construction of an image from the received signals is performed by the software of the MRI system.
The insertion of metallic wires into a biological organism (such as, e.g., catheters and guidewires) while the organism is in a magnetic resonance imaging environment poses potentially deadly hazards to the organism through excessive heating of the wires. In some studies, heating to a temperature in excess of 74 degrees Centigrade has created such hazards; see, e.g., an article by M. K. Konings, et al., in “Catheters and Guidewires in Interventional MRI: Problems and Solutions”, MEDICA MUNDI 45/1 March 2001.
The Konings et al. article lists three ways in which conductors may heat up in such environments: 1) eddy currents, 2) induction loops, and 3) resonating RF transverse electromagnetic (TEM) waves along the length of the conductors. It is disclosed in this article that: “Because of the risks associated with metal guidewires, and catheters with metal conductors, in the MRI environment, there is an urgent need for a non-metallic substitute, both for guidewires and for signal transfer.” The authors further propose the use of “ . . . a full-glass guidewire with a protective polymer coating . . . . ”
However, the use of such “ . . . full glass guidewire . . . ” presents its own problems. Many medical devices (such as, e.g., guides wires, stents, etc.) require some degree of strength and flexibility that is not afforded by glass guidewires and that typically require the use of metal or metal alloys in the device. The implementation of glass guidewires, optical fibers, etc., solutions would require substantial retooling of the, for example, catheter manufacturing industry and is not a suitable solution for other medical instruments that a physician may wish to employ, e.g. guidewires, stents, etc, during a medical procedure within an MRI system.
Compositions adapted to assist in visualizing medical devices in magnetic resonance imaging are well known. Reference may be had, e.g., to U.S. Pat. No. 6,361,759, the entire disclosure of which is hereby incorporated by reference into this specification. This patent describes and claims: “A coating for visualizing medical devices in magnetic resonance imaging, comprising a complex of formula (II): P-X-J-L-Mn+ (II), wherein P is a polymer, X is a surface functional group selected from the group consisting of an amino group and a carboxyl group, L is a chelate, M is a paramagnetic ion, n is an integer that is 2 or greater and J is the linker or spacer molecule and J is a lactam.”
U.S. Pat. No. 4,731,239 discloses and claims: “A method for nuclear magnetic resonance (NMR) imaging of a patient comprising, prior to the NMR imaging of a patient, administering to said patient ferromagnetic, paramagnetic or diamagnetic particles effective to enhance an NMR image.”
U.S. Pat. No. 4,989,608 discloses and claims: “A device which is specifically useful during magnetic resonance imaging of body tissue comprising: a flexible member of resinous material adapted to be inserted in the body tissue, the flexible member having ferromagnetic particles embedded therein at a concentration of about 0.001% to about 10% by weight of the material wherein, under magnetic resonance imaging, the flexible member exhibits characteristics which differ substantially from characteristics of the body tissue so that the visibility of the flexible member under magnetic resonance imaging is substantially enhanced, resulting in the flexible member being distinguishable from adjacent tissue as a dark area in brighter tissues and as a bright area in darker tissues, said member being free of elements which tend to degrade the overall quality of magnetic resonance images of the body tissue.” At column 2 of this patent, it is disclosed that: “Ferromagnetic particles in general can cause magnetic field artifacts (MRI signal voids, with adjacent very bright signal bands, hereinafter called ‘imaging artifacts’ which are considerably larger than the size of the particle.” The entire disclosure of this patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 5,154,179 discloses and claims: “1. A catheter which is specifically useful during a magnetic resonance imaging of body tissue comprising: a contrast agent; a flexible tubular member having a first lumen with an additional lumen positioned therein, the additional lumen retaining the contrast agent therein; the flexible tubular member being made of resinous material and adapted to be inserted in the body tissue, the flexible tubular member having ferromagnetic particles embedded therein at a concentration of about 0.001% to about 10% by weight of the material wherein, under magnetic resonance imaging, the flexible member exhibits characteristics which differ substantially from characteristics of the body tissue so that the visibility of the flexible member under magnetic resonance is substantially enhanced, resulting in the flexible member being distinguishable from adjacent tissue as a dark area in brighter tissues and as a bright area in darker tissues, said member being free of elements which tend to degrade the overall quality of magnetic images of the body tissue.” In the device of this patent, a ferromagnetic material was extruded into plastic as the plastic was being extruded to form the flexible tubular member. The entire disclosure of this United States patent is hereby incorporated by reference in to this specification.
U.S. Pat. No. 5,462,053 discloses and claims: “1. A contrast agent adapted for magnetic resonance imaging of a sample, said contrast agent comprising a suspension in a medium acceptable for magnetic resonance imaging of (a) coated particles of a contrast agent possessing paramagnetic characteristics and (b) coated particles of a contrast agent possessing diamagnetic characteristics, each of said coatings being selected from a group of materials which [I] renders said coated particles (a) and (b) substantially compatible with and substantially biologically and substantially chemically inert to each other and the environments to which said contrast agent is exposed during magnetic resonance imaging and [II] which substantially stabilizes said suspension; the nature of each of said coatings and the relative amounts of (a) and (b) in said suspension being such that the positive magnetic susceptibility of (a) substantially offsets the negative magnetic susceptibility of (b) and the resulting suspension has substantially zero magnetic susceptibility and, when employed in magnetic resonance imaging, results in the substantial elimination of imaging artifacts.” The entire disclosure of this United States patent is hereby incorporated by reference into this specification. In column 1 of this patent, it is disclosed that: “It is well known to enhance NMR . . . images by . . . introducing into the sample to be imaged ferromagnetic, diamagnetic, or paramagnetic particles which shadow the image produced to intensity and contrast the image generated by the NMR sensitive nuclei. See, for example, the disclosures of U.S. Pat. Nos. 4,731,239; 4,863,715; 4,749,560; 5,069,216; 5,055,288; 5,023,072; 4,951,674; 4,827,945; and 4,770,183 . . . .”
U.S. Pat. No. 5,744,958 discloses and claims: “A magnetic resonance imaging system, including: an imaging region and a means for generating a magnetic resonance image of a target object in the imaging region, said magnetic resonance image including an image of the target object, wherein the means for generating the magnetic resonance image includes means for producing an RF field having an RF frequency in the imaging region; and an instrument for use with the target object in the imaging region, said instrument including: an electrically non-conductive body, sized for use with the target object in the imaging region; and an electrically conductive, ultra-thin coating on at least part of the body, the coating being sufficiently thick to cause the instrument to be positively shown in the magnetic resonance image in response to presence of the instrument in the imaging region with the target object during generation of the magnetic resonance image, wherein the coating consists of material having a skin depth with respect to said RF frequency and the coating has a thickness less than the skin depth.” At column 4 of this patent, it is disclosed that: “The present invention is based on the inventor's recognition that an electrically conductive, ‘ultra-thin’ coating (a coating whose thickness is less than or of the same order of magnitude as the coating's skin depth with respect to its electrical and magnetic properties and the frequency of the RF field in an MRI system) on an instrument can cause the instrument to create just enough artifact to be visible when imaged by an MRI system, but not so much artifact as to obscure or distort unacceptably the magnetic resonance imaging of a target (e.g., human tissue) also being imaged by the MRI system. In other words, the invention controls the artifact in such a way as to make the instrument visible but not appreciably distort the tissue structures being imaged by the MRI. An ultra-thin coating on an instrument embodying the invention typically has a thickness of on the order of hundreds or thousands of Angstroms.” The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 6,203,777 describes and claims: “In a method of contrast enhanced nuclear magnetic resonance diagnostic imaging which comprises administering into the vascular system of a subject a contrast enhancing amount of a nuclear magnetic resonance imaging contrast agent and generating an image of said subject, the improvement comprising administering as said contrast agent composite particles comprising a biotolerable, biodegradable, non-immunogenic carbohydrate or carbohydrate derivative matrix material containing magnetically responsive particles, said magnetically responsive particles being of a material having a Curie temperature and said composite particles being no larger than one micrometer in size.” The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
United States published patent application 2002/0176822 discloses and claims: “A magnetic resonance imaging system, comprising: a magnetic resonance device for generating a magnetic resonance image of a target object in an imaging region; and an instrument for use with the target object in the imaging region, said instrument including a body sized for use in the target object and a polymeric-paramagnetic ion complex coating thereon in which said complex is represented by formula (I): P—X-L-Mn+ (I) wherein P is a polymer, X is a surface functional group, L is a chelate, M is a paramagnetic ion and n is an integer that is 2 or greater.” The entire disclosure of this United States patent application is hereby incorporated by reference into this specification.
None of the prior art compositions or coatings appear to be adapted to both facilitate MRI imaging while simultaneously protecting biological tissue within a living organism from the adverse effects of the MRI electromagnetic wave. By way of illustration, some of the adverse effects include heating of tissue in contact with an implanted, conductive medical device, and voltages induced across tissue near or contiguous with leads of implanted medical devices.
It is an object of this invention to provide an assembly for protecting biological tissue from the adverse effects of heating during MRI scanning while simultaneously facilitating MRI imaging.